Fifth Wheel RV Build

Contrails

New member
Joined
Feb 1, 2021
Messages
7
Hey all! 🖐

So I initially posted in the welcome forum some initial questions about starting a build for my new fifth wheel, and was quickly directed to start a thread else where; so here I am. I have a moderate level of knowledge of electrical systems (I've wired two homes with standard US wiring practices) however my knowledge of battery packs/technology is pretty low to none. So I'm here looking to learn and start the process of creating a system that will hopefully alleviate the need for a generator for the fifth wheel. I'll outline some goals/needs below and hopefully get some people chiming in with guidance.

First up; I'm NOT looking for everyone to solve my problems here, I'd rather learn and research myself, but I don't even know where to begin. Some of the jargon used is like woah what did he just say? I can dig into the YouTubes, but I'd prefer to not waste time on tons of useless videos. I'd love some suggestions of good places to go to learn or books to buy and read.

For the set up I am imagining...

  • I'd like to be able to run my fifth wheel for 4-5 days with a small solar set up, or possibly charge from either my truck (I have dual 220amp alternators on it) or a smaller generator.
  • I need to covert the battery power back to 110V with the hope of running the following:
    • Fridge
    • Electric fireplace
    • coffee pot / instapot / crockpot
    • CPAP machine
The RV has a large bay that is meant to hold a Cummins ONAN 5500 generator in it, so I have a decent space that is heat shielded/vented and has good structural support.

From my other thread, it looks like LiFePO4 cells are the way to go for this set up. I'm looking at these as a potential candidate:


I guess initially I need to design a basic system that:
  • Allow charging by multiple methods (solar, truck alternators, shore power)
  • conversion of the battery bank to 110
Simple right?
 

Redpacket

Well-known member
Joined
Feb 28, 2018
Messages
1,263
Any reason you're not looking at large prismatics from day one?
Might be more robust with better capacity?

This guys does lots of really good youtube videos of just this sort of thing:
Will Prowse
+ there's lots of info on RV electrics out there too.
 

ajw22

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Nov 16, 2018
Messages
573

italianuser

Member
Joined
Feb 25, 2020
Messages
84
First thing I would describe in details all devices you want to power on a daily basis, specifying for each wattage. Consider that nominal wattage on the device isn't how much it actually uses because PSU are usually over dimensioned (for e.g. my notebook has an 65W power supply but actually only consumes 20-25Wh when battery is charged and 40Wh when charging battery).

This document will be the starting point for dimensioning your system, something like this:

1) Notebook Lenovo: 40W, 4 hours/day = 160Wh
2) Desk lamp: 6W, 6 hours/day = 36Wh
3) ...
Devices Total/Day: 196Wh
Inverter: 10% + 10W
Total: 226Wh
Total 5 days: 1130Wh

(You could add a 10% for dispersion, cable resistance, ecc)

After that I can surely dimension battery pack. Using a lead-acid battery I consider available capacity at 50%, under that I'm killing the battery.

So, a 12V 200Ah acid lead battery gives me 12V x 200Ah = 2400Wh, at 50% is 1200Wh. And that's my battery capacity.

Now how to charge it is your choice. With solar panels it's quite an easy calculation to get an approximate value. 4 x 100W 12V panels would give you 400Wh when fully exposed to sun. Where I am I daily get 5 hours of full light in summer, that is: 5 x 400Wh = 2000Wh, so I could charge a 200Ah battery in about one day.

You can choose the voltage for your system depending on your needs, I suppose you'd use something from 24V upwards. Wattage calculation is always valid.

jes
 
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OffGridInTheCity

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Joined
Dec 15, 2018
Messages
1,462
Was wondering the same. A pre-built 12V LiFePO4 battery like this one would save a lot of time and effort.
And it's not that much more expensive, if you include costs for BMS, box, etc. Don't like that there's no cell level voltage display, though.
Agree with @ajw22's advice here. It took me several months to build and equivalent 12v@1000ah part-time. If you have lots of time and interest then maybe build it yourself from raw cells. Also there are in-between options such as rebuilding from modules like this - https://batteryhookup.com/products/lg-14s-48v-2-2kwh-lithium-ion-module If you go 24v or 48v there are 24/48 -> 12v converters. I use a 48v -> 12v@80a converter for my 12v subsystem.
 

Contrails

New member
Joined
Feb 1, 2021
Messages
7
Well, FWIW I don't know what I don't know yet.

I had seen pictures of power walls built for fairly cheap (that's relative) that powered whole houses using 18650s and thought it might be a good idea when compared to the cost of a battle born battery or one linked above from Amazon. Plus it looked cool :cool:

Regardless I'm after bang for the buck and robustness. Ideally, I'd like to come off shore power at home, drive a couple of hours to the campsite, spend 4-5 days there off just battery power (no need to charge), and return home. Obviously this will take some power management and I cant/wont run things 100% of the time, but for something in the 500-1000AH range, I would think it's doable.

For instance, I could use these batteries:

https://www.aliexpress.com/item/1005001715950047.html?spm=a2g0o.productlist.0.0.448d22ffvjWPpY&algo_pvid=9dabf2a8-c1c4-4ff0-a382-3a1cfadc9d1e&algo_expid=9dabf2a8-c1c4-4ff0-a382-3a1cfadc9d1e-0&btsid=0b0a555816122497494176470e6460&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_ Purchase 8 of them and have a 560 AH battery for $918 vs $1300 for a 300 AH from Amazon (30% savings).

I had found this gentleman last night on youtube, and I liked his milkcrate setup. I would think that I could do something similar in the front bay of the fifthwheel.



The 48V system is in interesting prospect with the modules (I like that they're rack mountable), however, I'm totally out of depth there and wouldn't even know where to begin. I did some initial math on it, and each one of those battery packs yields about 45 amp hours, so so make an equivalent bank, I'd need 12 of them (~ $2600). The price of that setup might be a no go unless I'm missing something.
 

OffGridInTheCity

Well-known member
Joined
Dec 15, 2018
Messages
1,462
You're on the right track.

I think you have this part......
A key thing to get your head around is ah / voltage / watthours (overall power). Let's say you have 4 x 12v batteries at 100ah each. IF....
1) You hook then in parallel (1s4p) - you have 12v@400ah = 4800watt-hours.
2) You hook 2 in series + 2 in parallel (2s2p) - you have 24v@200ah = 4800watt-hours.
3) You hook them in series (4s1p) - you have 48v@100ah = 4800watt-hours.
All 3 = the same amount of 'power' (agility run things for X hours) but as the voltage rises the amps go lower which means smaller wire.
Let's say you want a 3000w inverter.....
1) 3000w inverter @ 12v = 3000/12 = 250amp wiring. This is very large wire!
2) 3000w inverter @ 24v = 3000/24 = 125amp wiring. Pretty big.
3) 3000w inverter @ 48v = 3000/48 = 62amp wiring.

The other difference between 12v/24v/48v is the equipment. They make lots of inverters for 12v or 24v or 48v - no problem. The other issue is to larger than 12v you need an XXv -> 12v converter.
-------------------------------

I would estimate you can save up to 80% by buying/building your own DIY battery. There are 2 basic choices....
1) Cells - you put them in to packs and then hook the packs together with a BMS and some way to mount it all in the RV. **This is what I did.
2) Large Format such as Prismatics - much less hookup but can be more expensive.
3) Whole Batteries - such as @awj22 gave a link to or DIY ones that I pointed you to.

The decision as to which way to go get's personal and complex and is a function of $$ and Time and Effort and Fun(or not). If you go 24v or 48v you can do either LifePo4 or Lithium-Ion - wider choice. If you go 12v you need to stick with LifePo4 because ltihium-Ion voltages (3 in series or 4 in series) just aren't compatible with the 12v range.

Besides $ and Time and Effort and Fun(or not) it may be important to look to the future - how long do you want them to last. LifePo4 is the best chance of extra long life compared to Lithium-ion.
----------------------------------

All of this is why @awj22 pointed you to 'ready to go', pretty well priced 12v LifePo4. 2 of those would be $2600.

It cost me 3-4 months + $1500 to build that 14s88p lithium-ion battery in my trailer - e.g. 48v@260ah = 12,480watt-hours. That's twice the power of the link above and pretty low $ (in my opinion) but it was based on 1300 lithium ion cells at $0.60c each + I'm well established to do this with all the knowledge, equipment, process etc.

In comparison that would be $750 compared to $2600 - but you need to learn several new skills + spend significant personal time. In addition, it can take a few months to find am absolute 'best buy' cell price (such as $0.60c/cell). :)

If you want to start looking at batteries / cells etc - hear are the 2 places that I have highest success/trust over time and can recommend that you'll get what you purchase:
For 12v DIY battery - here's brand new Battery Hookup lifePo4 example - https://batteryhookup.com/products/new-battery-hookup-lifep04-32650-3-2v-5000mah-cells You would need 400 of these for 12v@500ah. 400 * $2.99 = $1,196.

For 12v DIY battery - here's a Battery Clearing House 2nd-hand LifePo4 Cell example to give you an idea - https://www.batteryclearinghouse.co...r26659m1b-lifep04-3-3v-2500mah-in-3-emc-packs
You would need about 1,000 of these for 12v@500ah. 7 x $140 = $980.

**This is just talk/examples. Do not let 'my talk' push you in any particular direction - that's not my intent. All my work has been individual cells - so that's what I know and I'm just sharing that. I know others have done really well with EVs and other approaches. :)
 
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Contrails

New member
Joined
Feb 1, 2021
Messages
7
You're on the right track.

I think you have this part......
A key thing to get your head around is ah / voltage / watthours (overall power). Let's say you have 4 x 12v batteries at 100ah each. IF....
1) You hook then in parallel (1s4p) - you have 12v@400ah = 4800watt-hours.
2) You hook 2 in series + 2 in parallel (2s2p) - you have 24v@200ah = 4800watt-hours.
3) You hook them in series (4s1p) - you have 48v@100ah = 4800watt-hours.
All 3 = the same amount of 'power' (agility run things for X hours) but as the voltage rises the amps go lower which means smaller wire.
Let's say you want a 3000w inverter.....
1) 3000w inverter @ 12v = 3000/12 = 250amp wiring. This is very large wire!
2) 3000w inverter @ 24v = 3000/24 = 125amp wiring. Pretty big.
3) 3000w inverter @ 48v = 3000/48 = 62amp wiring.

The other difference between 12v/24v/48v is the equipment. They make lots of inverters for 12v or 24v or 48v - no problem. The other issue is to larger than 12v you need an XXv -> 12v converter.
-------------------------------

I would estimate you can save up to 80% by buying/building your own DIY battery. There are 2 basic choices....
1) Cells - you put them in to packs and then hook the packs together with a BMS and some way to mount it all in the RV. **This is what I did.
2) Large Format such as Prismatics - much less hookup but can be more expensive.
3) Whole Batteries - such as @awj22 gave a link to or DIY ones that I pointed you to.

The decision as to which way to go get's personal and complex and is a function of $$ and Time and Effort and Fun(or not). If you go 24v or 48v you can do either LifePo4 or Lithium-Ion - wider choice. If you go 12v you need to stick with LifePo4 because ltihium-Ion voltages (3 in series or 4 in series) just aren't compatible with the 12v range.

Besides $ and Time and Effort and Fun(or not) it may be important to look to the future - how long do you want them to last. LifePo4 is the best chance of extra long life compared to Lithium-ion.
----------------------------------

All of this is why @awj22 pointed you to 'ready to go', pretty well priced 12v LifePo4. 2 of those would be $2600.

It cost me 3-4 months + $1500 to build that 14s88p lithium-ion battery in my trailer - e.g. 48v@260ah = 12,480watt-hours. That's twice the power of the link above and pretty low $ (in my opinion) but it was based on 1300 lithium ion cells at $0.60c each + I'm well established to do this with all the knowledge, equipment, process etc.

In comparison that would be $750 compared to $2600 - but you need to learn several new skills + spend significant personal time. In addition, it can take a few months to find am absolute 'best buy' cell price (such as $0.60c/cell). :)

If you want to start looking at batteries / cells etc - hear are the 2 places that I have highest success/trust over time and can recommend that you'll get what you purchase:
For 12v DIY battery - here's brand new Battery Hookup lifePo4 example - https://batteryhookup.com/products/new-battery-hookup-lifep04-32650-3-2v-5000mah-cells You would need 400 of these for 12v@500ah. 400 * $2.99 = $1,196.

For 12v DIY battery - here's a Battery Clearing House 2nd-hand LifePo4 Cell example to give you an idea - https://www.batteryclearinghouse.co...r26659m1b-lifep04-3-3v-2500mah-in-3-emc-packs
You would need about 1,000 of these for 12v@500ah. 7 x $140 = $980.

**This is just talk/examples. Do not let 'my talk' push you in any particular direction - that's not my intent. All my work has been individual cells - so that's what I know and I'm just sharing that. I know others have done really well with EVs and other approaches. :)

I think you've got me convinced to step up to a 24v, or possibly a 48v system. From some initial youtubin, it seems that the higher voltage systems are a bit safer (running less amps), components (chargers/converters/controllers/wire) are cheaper. Are there any disadvantages to going to a higher voltage?
 

chuckp

Active member
Joined
Jul 29, 2018
Messages
384
personally from an electric shock safety point. If you go with a 48V build, have a max charge voltage of around 56/57V.
Under 60V DC will still hurt if you get a belt off of it, but above 60V you starting to get onto a serious electric shock if something where to go wrong. Also the HV inverters/chargers etc start to go up a fair bit in price. And like OffGridInTheCity said you’ll have more options available to you.
 
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